in my Oil/Air multiphase flow calculation with a rotating domain (only 1° is modeled) i want to add gravity in the radial direction z. The rotating axes is x. If I set the buoyancy model and use

x=0 [m/s^2]
y=0 [m/s^2]
z=9.81 [m/s^2]

the solver automatically counter-rotates the relative Frame gravity vector and so the vector components y and z Change with the Rotation angle. How can i avoid this? Could i use a subdomain and define a momentum source in order to model the gravity?
What do i have to add there? The unit is [Nm]

i think gravity is the wrong word. In my rotational domain (which could be imagined as a rotational couette flow) I get a state where oil droplets are surrounded by air, so they hover in space with no velocity. With the additional body force I want to get them out of the Domain in the radial z direction.

I still tried to make the gravity (buoyancy) vector as a function of time but the calculation is not exact because my rotational speed also changes with time.

So what I have to add in the momentum source term to apply a Body acceleration in z?